The present invention relates, in general, to reference voltage circuits, and more particularly, to bandgap reference voltage circuits.
A bandgap reference voltage circuit provides a temperature and supply independent output reference voltage in analog integrated circuits such as filters, analog-to-digital converters, and digital-to-analog converters. The reference voltage is generated by weighting a voltage having a positive temperature coefficient and weighting a voltage having a negative temperature coefficient such that the sum of the two voltages is the reference voltage having a zero temperature coefficient. A common technique for generating the voltage having the positive temperature coefficient is to operate two bipolar transistors at different current densities, thereby generating a delta base-to-emitter voltage (.DELTA.V.sub.BE) having a positive temperature coefficient. The voltage having the negative temperature coefficient is generated from the V.sub.BE voltage of a third bipolar transistor. As those skilled in the art are aware, a V.sub.BE voltage inherently has a negative temperature coefficient. Generation of voltages having positive and negative temperature coefficients is further described in U.S. Pat. No. 3,887,863, titled "SOLID-STATE REGULATED VOLTAGE SUPPLY" and issued to Brokaw on Jun. 3, 1975, and which is hereby incorporated herein by reference.
A drawback of prior art bandgap reference circuits is that their output voltage levels are dependent upon the weighting factors that provide a zero temperature coefficient output reference voltage. In addition, the prior art bandgap reference circuits require a supply voltage of at least 1.5 volts, which may be unsuitable for low power circuit applications.
Accordingly, it would be advantageous to have a method and a means for generating a programmable bandgap output reference voltage. It would be of further advantage to have a bandgap reference circuit capable of operating with a supply voltage of less than 1.5 volts.